This invention relates to apparatus for sensing short circuit faults in alternating current supply lines and, especially although not exclusively, to the protection of alternating current supplies when arranged for supplying direct-on-line starting three phase squirrel cage induction motors.
The present invention has arisen from the consideration of a particular problem which is met in the use of direct-on-line starting three phase squirrel cage induction motors in coal mines but is of more general application.
The particular problem mentioned above is that of providing adequate protection of a three phase supply against short circuits between the phase lines or to a neutral point while permitting direct-on-line starting of three phase squirrel cage induction motors connected to the three phase supply. That this is a problem is due to the fact that the value of the starting current drawn in each line by such a motor during direct-on-line starting has several times, for example four to ten times, the amplitude of the current drawn when the motor is running under full load conditions and consequently no protection based merely upon sensing of the amplitude of the currents drawn is practicable for short circuit faults which are drawing currents which are of less amplitude than that of the motor starting current. While a conventional motor overload protection will respond to short circuit current in this range, its inherent time delay will be long enough to permit damage to be done to the supply and possibly also the motor.
Direct-on-line starting is used for three phase squirrel cage induction motors in coal mines and other hazardous environments because simplicity in operating equipment is found to be essential there. However, the risk that supply lines may be damaged or cut is always present and it is also essential that the consequent risk of fire or explosion started by electrical fault arcing should be minimised and eliminated if at all possible. Hence there is a requirement for means which will isolate the source of three phase supply from any short circuit faults on the supply lines.